1. Field of the Invention
The present invention relates to a high strength and low yield ratio steel for structure having excellent characteristics such as low temperature toughness and a manufacturing method thereof, and more particularly, to a high strength steel satisfying excellent main characteristics such as low temperature toughness and low yield ratio, both of which are required for steel for structure, by employing method using a matrix structure of steel as bainitic ferrite and granular bainite structures and using dual phase having high hardness, and a manufacturing method thereof.
2. Description of Related Art
Structures such as buildings and bridges mainly require high strength due to their high loads. Also, the total weight of used steel tends to be reduced with continued demand for reduction in the cost of construction materials used to build constructional structures. Therefore, there has been an increasing demand for an increase in strength of steel constituting these constructional structures.
However, since the steel has a problem in that its properties such as low temperature toughness may be often deteriorated with its increasing strength, a lot of high strength steels for structure has poor low temperature toughness. The low temperature toughness is the measure on how long steel ensures brittle fracture at ultra-low temperature, and steels having poor low temperature toughness have a problem in that brittle fracture may occur easily in the steels when the steels are used in severe low-temperature regions such as extreme regions, which leads to the limitations on use environments of the steels. A ductile-brittle transition temperature (DBTT curve) is generally used as the measure of low temperature toughness.
Also, the increase in strength of steel results often in the increase in a yield ratio that is a ratio of yield strength to tensile strength. Then, the increase in the yield ratio reduces the stress difference from a time point (yield point) that plastic deformation of steel occurs to a time point that fracture of steel occurs. Therefore, since buildings have little preparation time to prevent destruction of the buildings by absorbing energy through their deformations, it is difficult to secure the safety of constructional structures when the constructional structures are exposed to tremendous external forces such as earthquakes.
Therefore, the steels for structure should necessarily have low temperature toughness and low yield ratio, both of which are maintained over certain levels.
As one of alternative technologies to secure a low yield ratio of steel, there is a method for enhancing low yield ratio of steel by selecting suitable alloying elements of the steel and suitably adjusting rolling conditions. This technology is to improve tensile strength of steel, and thus to secure a low yield ratio of steel by adjusting alloying elements to suitable ranges, finish cooling temperature below 500° C. to form a bainitic ferrite structure, heat-treating the bainitic ferrite structure at an intercritical temperature range of 700 to 760° C. to form austenite between bainite laths, and slowly cooling the austenite to obtain a MA (Martensite or/and residual austenite) structure.
In order to make a microstructure of steel as a bainitic ferrite structure, the finish cooling temperature should, however, be adjusted to a temperature below Bf temperature that is a bainite transformation finish temperature. In this case, problems associated with low productivity may occur in production line. Also, the process of obtaining a MA structure by the heat-treatment of the bainitic ferrite structure at the intercritical temperature range after the rolling process has problems associated with the delayed supplies of the products, the increased manufacturing cost, the reduced productivity, etc.
Therefore, there is a demand for development of steel having high productivity as well as satisfying requirements such as high strength characteristics, low temperature toughness characteristics and low yield ratio.
The present invention is designed to solve the problems of the prior art, and therefore it is an object of the present invention to provide a high strength steel satisfying all characteristics such as low temperature toughness and low yield ratio.
Also, it is another object of the present invention to provide a method for manufacturing a high strength steel satisfying all characteristics such as low temperature toughness and low yield ratio.